Contemporary electronics and portable electronic devices commonly include one or more microphones, and as more features are being introduced, more than one microphone may be included for complex audio processing. One such microphone is the electret condenser microphone (ECM) that includes a capacitive sensing plate and a field effect transistor (FET) amplifier. The FET amplifier can be in an integrated circuit (IC) die located within the microphone package enclosure. The IC die may additionally include an analog to digital converter (ADC) for digital microphone applications.
More recently, micro-electro-mechanical systems (MEMS) microphones have been introduced. Similar to an ECM, a MEMS microphone may feature capacitive sensing with a fixed diaphragm. In addition to an amplifier and ADC, a MEMS IC die may include a charge pump to bias to diaphragm.
ECM and MEMS microphone packages include a sound inlet, or hole, adjacent the capacitive sensing plate or membrane for operation, e.g., to allow the passage of sound waves that are external for the package. A particle filter may be provided in order to mitigate the impact of particles on operation. Sound waves entering through the sound inlet exert a pressure on the capacitive sensing plate or membrane, and an electrical signal representing the change a capacitance is generated.
Recently MEMS microphones have been adapted for use in mobile electronic devices such as smartphones, music players and mobile computers. In portable devices, however, the interference from unwanted environmental sounds (e.g., noise) becomes more problematic for audio sensing. Many of the technologies developed for eliminating or cancelling unwanted sounds use conventional microphones that detect sound through air. Such systems, however, may face challenges when it comes to distinguishing between desirable sounds falling within frequency ranges typical of unwanted sounds (e.g., low frequency ranges).